Wireless receivers are required to align their receiver frequency clock with that of the transmitter with which they are communicating. Wireless receivers commonly use a crystal oscillator as the clock source. Variability of the frequency derived from the clock at the receiver can result in errors between the frequency transmitted and the demodulated received signal, resulting in a carrier frequency error. In addition there can also be a Doppler shift in the frequency due to the rate of change in distance between the transmitter and receiver. This frequency error can affect the received signal in several ways. One particular problem is that if the signal strength of the received signal is low. In order to account for a carrier frequency error, the detection bandwidth must be wide enough to cater for not only the bandwidth of the wanted signal, but also for the maximum possible frequency error. The level of noise at the input of a receiver is proportional to the bandwidth and hence, in order to detect a low level signal, it is desirable to keep the bandwidth to a minimum. It is common in digital communications systems to use a fixed preamble which can be used to detect the start of a transmission and for frequency correction.
In many wireless systems, time division multiplexing (TDM) is used as a method to transmit and receive signals. TDM is used primarily for digital signals where the time domain is divided into several time slots, usually of fixed durations, which are termed sub-channels. During each time slot, a data block is transmitted that is addressed to a particular station. TDM is often used in satellite based communications where the satellite communicates with a number of ground based stations. At the ground based station the reception of the transmissions from the satellite is therefore seen as a series of burst signals, coincident with one or more time slots. In such a system, the satellite is usually equipped with a very accurate time clock, but ground based stations, especially in the case of mobile stations, will tend to use a crystal oscillator reference.
Mobile stations can experience a range of signal strength conditions ranging from a clear line of sight, to one where there is no line of sight and the signal is subject to varying degrees of obstruction losses. As the signal level drops, the signal to noise ratio (SINR) decreases. Also, the effects of any co-channel interference will become more prominent. As the signal to noise and interference ratio (SNIR) drops and noise and interference become more prominent, the result is that noise spikes will occur within the receiver detection bandwidth. Even if the transmitter is sending regular signals that can be used to correct and maintain the frequency accuracy, the mobile station can be in a location or condition where it can lose the signal and hence can undergo relatively long periods with no received signal. Such an example would be when a mobile ground station is taken indoors and it loses the signal from the satellite. Indeed, in many communication satellite systems, the received satellite signal is generally low and the mobile station is generally operating under low SNIR conditions. When the signal is lost for significant periods, the relative clock drift between the ground-based station and the satellite will result in a higher SNIR to be required in order to capture the signal. A typical crystal clock reference will have an accuracy of ±1 ppm and stability over the operating temperature range of ±20 ppm. Ageing and voltage variations will also affect the absolute accuracy. After long periods of no reception, due to the device being switched off, being in a location that obstructs the transmitter or located indoors, the relative clock drift may be significant and it is usually necessary to cater for up to 1 ppm possible drift. For a satellite system that has carrier frequencies at 1.5 GHz or higher, 1 ppm represents a frequency error of 1500 Hz.
One satellite system of interest is one that meets the GMR1 specification. One example of such a satellite system is the commercial Thuraya® satellite system which operates in the L band at downlink carrier frequencies of 1525-1559 MHz. In a GMR1 satellite system there are three different categories of control channels. The broadcast control channel is a downlink channel that includes the Frequency Correction Channel (FCCH) which is intended for frequency correction, in practice, using the scheme provided by the FCCH, it requires a certain SNIR to function and it is highly desirable to improve this estimation performance.